Background-audio handoff for video + repository/player refactor

Hand video playback off to a native audio-only stream when the app is
backgrounded or locked, with no on-device video decode (UR-040). Adds
player_enter/exit_background_audio commands, an audio-only stream URL
for video items across the repository layer, and the frontend handoff
state machine wired into VideoPlayer. Includes accompanying
repository/offline/player refactoring and regenerates the traceability
matrix.

src-tauri/src/player/media.rs

@@ -138,8 +138,12 @@ impl MediaItem {
     /// Get the Jellyfin item ID if available
     pub fn jellyfin_id(&self) -> Option<&str> {
         match &self.source {
-            MediaSource::Remote { jellyfin_item_id, .. } => Some(jellyfin_item_id),
-            MediaSource::Local { jellyfin_item_id, .. } => jellyfin_item_id.as_deref(),
+            MediaSource::Remote {
+                jellyfin_item_id, ..
+            } => Some(jellyfin_item_id),
+            MediaSource::Local {
+                jellyfin_item_id, ..
+            } => jellyfin_item_id.as_deref(),
             MediaSource::DirectUrl { .. } => None,
         }
     }
@@ -151,9 +155,7 @@ impl MediaItem {
     pub fn playback_url(&self) -> String {
         match &self.source {
             MediaSource::Remote { stream_url, .. } => stream_url.clone(),
-            MediaSource::Local { file_path, .. } => {
-                file_path.to_string_lossy().to_string()
-            }
+            MediaSource::Local { file_path, .. } => file_path.to_string_lossy().to_string(),
             MediaSource::DirectUrl { url } => url.clone(),
         }
     }